Biaxially oriented polyester film and process for production thereof

ABSTRACT

A biaxially oriented polyester film wherein the ratio (=I MD  /I ND ) of the peak intensity (I MD ) in the longitudinal direction to the peak intensity (I ND ) in the thickness direction determined at 1615 cm -1  by laser Raman scattering method is 6 or more; and a process for production of a biaxially oriented polyester film which comprises the steps of controlling the ratio (A/B) of the maximum thickness (A) of the edge portion of a cast film to the thickness (B) of the central portion in the transverse direction thereof in the range of 2 to 6, biaxially stretching the cast film, and controlling the peak intensity ratio of the biaxially oriented film determined by the laser Raman scattering method in the above-described range. The biaxially oriented polyester film thus produced has a specific orientation in the longitudinal direction, and therefore has great strength in the longitudinal direction and small irregularity in properties such as thickness or birefringence.

This application is a 371 of PCT/JP95/01702, filed Aug. 28, 1995.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a biaxially oriented polyester film anda process for producing the same. More particularly, the presentinvention relates to a specified biaxially oriented polyester film whichhas small irregularities in qualities such as thickness or widthwiseproperty and wherein the productivity with respect to breakage and yieldis improved. The principal orientation axis is present in thelongitudinal direction and the film is more oriented in the longitudinaldirection. And a process for producing the film is also disclosed.

BACKGROUND ART OF THE INVENTION

In plastic films, it is possible to continuously produce a film having agreat area which cannot be achieved by other materials. Utilizing thecharacteristics such as strength, durability, transparency, flexibilityand separation property between surface side and back surface side,plastic films are used in fields such as agricultural, package andarchitectural fields which require large amounts of plastic film.Specifically, because biaxially oriented polyester films have excellentmechanical, thermal, electrical and chemical properties, they are usedfor various fields, and particularly, the usefulness thereof as basefilms of magnetic tapes is unrivaled by films made from other materials.Recently, such base films have been required to be thinner in order tolighten and miniaturize equipment, enable longer recordings, andtherefore, the base films have been required to be further strengthened.

Further, in the fields of heat transfer ribbons, capacitors andthermo-stencil printing plates, thin films have been strongly required,and similarly the films used have been required to be furtherstrengthened.

As a process for strengthening a biaxially oriented polyester film,generally known is a so-called longitudinal re-stretching process forre-stretching a biaxially stretched film in the longitudinal directionand providing a high lengthwise strength to the film (for example,JP-B-SHO 42-9270, JP-B-SHO 43-3040, JP-A-SHO 46-1119 and JP-A-SHO46-1120). Further, in a case where a film is required to be furtherstrengthened in the transverse direction as well as in the longitudinaldirection, there is a so-called longitudinal and transversere-stretching process for re-stretching the film in the transversedirection after the longitudinal re-stretching (for example, JP-A-SHO50-133276 and JP-A-SHO 55-22915).

However, even in such a longitudinal re-stretching process, although theprincipal orientation axis is present in the longitudinal direction, theorientation of the whole of the film is not so great, and irregularityin thickness and irregularities in properties in the transversedirection are not greatly improved.

Moreover, in a process having the above-described longitudinalre-stretching process, because generally a thin film having a largewidth is stretched by rollers, if an edge portion of the film is thin,the neck down at the time of stretching is violent, thereby causingserious problems such as deterioration of irregularity in thickness,irregularity in other properties, and generation of scratches. In orderto avoid these problems, generally edges of a cast film are formed thickand a holding force is provided thereto for preventing the neck down.

In a case where longitudinal re-stretching is performed, a cast filmmust be formed so that a ratio (A/B) of the maximum thickness (A) of theedge portion to the thickness (B) of the central portion in thetransverse direction of the cast film is a value around 10, although theratio to be controlled depends upon the thickness of the centralportion.

However, if the difference between the thickness of the edge portion andthe thickness of the central portion is great as described above,although a neck down can be lessened problems occur in that the edgeportion having a large thickness is insufficiently pre-heated, therebycausing frequent film breakage, and irregularity in properties in thetransverse direction due to the temperature difference becomes great.Further, because the edge portions are finally trimmed away from a filmproduct portion, thick edge portions are not desired also from theviewpoint of yield.

DISCLOSURE OF THE INVENTION

Paying attention to problems originating from a longitudinalre-stretching, an object of the present invention is to provide abiaxially oriented polyester film in which a principal orientation axisis present in the longitudinal direction and the longitudinalorientation is increased without performing a longitudinalre-stretching, and which has less irregularity. More concretely, anobject of the present invention is to provide a biaxially orientedpolyester film wherein the edge portions can be thinly formed withoutlongitudinal re-stretching, the irregular thickness and the irregularproperties in the transverse direction can be reduced, the frequency offilm breakage can be decreased, the yield can be increased, which has aprincipal orientation axis in the longitudinal direction and which ismore oriented in the longitudinal direction; a process for production isalso disclosed.

To accomplish the above objects, a biaxially oriented polyester filmaccording to the present invention is characterized in that a ratio R(=I_(MD) /I_(ND)) of a peak intensity (I_(MD)) in the longitudinaldirection of the film to a peak intensity (I_(ND)) in the thicknessdirection of the film determined at 1615 cm⁻¹ by laser Raman scatteringmethod is not less than 6.

A process for producing a biaxially oriented polyester film according tothe present invention comprises the steps of controlling a ratio (A/B)of the maximum thickness (A) of an edge portion of a cast film to thethickness (B) of a central portion in the transverse direction of thecast film in the range of 2 to 6; stretching the cast film biaxially;and controlling a ratio R (=I_(MD) /I_(ND)) of the biaxially orientedfilm of a peak intensity (I_(MD)) in the longitudinal direction of thefilm to a peak intensity (I_(ND)) in the thickness direction of the filmdetermined at 1615 cm⁻¹ by laser Raman scattering method to be not lessthan 6.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a schematic view of an apparatus for determining a breakdownvoltage of a film.

THE BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, a ratio R (=I_(MD) /I_(ND)) of a peakintensity (I_(MD)) in the longitudinal direction to a peak intensity(I_(ND)) in the thickness direction determined at 1615 cm⁻¹ by laserRaman scattering method is a factor indicating an intensity of totalorientation in the longitudinal direction. This can be determined bymeasuring Raman scattered rays generated when a laser ray is applied toa film. In the Raman spectrum, the Raman band at 1615 cm⁻¹ belongs toC═C stretching vibration of a benzene ring (ν C═C), and the longitudinalorientation degree of the whole of the film can be determined bydetermining the ratio (=I_(MD) /I_(ND)) of the intensity in thelongitudinal direction to the intensity in the thickness direction.

In the present invention, in order to obtain a film having asufficiently great strength and in order to improve irregularity inthickness and irregularity in properties in the transverse direction,the intensity ratio R must be not less than 6. Preferably it is not lessthan 7. Further, it is preferred to be not less than 8 for use ofmagnetic materials.

In the biaxially oriented polyester film according to the presentinvention, an amorphous orientation coefficient f_(MD) in thelongitudinal direction of the film is preferably not less than 0.5 fromthe viewpoint of increase of strength, and more preferably it is notless than 0.7.

Although an F-5 value of an ordinary biaxially oriented polyester filmon the market is generally in the range of 11 to 13 kg/mm², in order toremarkably accomplish the purpose of lightening and miniaturizing inuses for base films of magnetic tapes, ribbons, capacitors andthermosensible stencil printing plates, it is effective to set the F-5value in the longitudinal direction higher, and it is preferably notless than 15 /mm², and more preferably not less than 16 /mm².

The polyester in the present invention is a polymer prepared from a dioland a dicarboxylic acid by condensation polymerization. The dicarboxylicacid is represented by terephthalic acid, isophthalic acid, phthalicacid, naphthalene dicarboxylic acid, adipic acid, sebacic acid, etc. Thediol is represented by ethylene glycol, trimethylene glycol,tetramethylene glycol, cyclohexanedimethanol, etc. Concretely, forexample, polymethylene terephthalate, polyethylene terephthalate,polypropylene terephthalate, polyethylene isophthalate,polytetramethylene terephthalate, polyethylene-p-oxybenzoate,poly-1,4-cyclohexylenedimethylene terephthalate andpolyethylene-2,6-naphthalate can be used. Of course, these polyestersmay be either homopolymer or copolymer. As the copolymerizationcomponent, for example, a diol component such as diethylene glycol,neopentyl glycol or polyalkylene glycol, or a dicarboxylic componentsuch as adipic acid, sebacic acid, phthalic acid, isophthalic acid or2,6-naphthalene dicarboxylic acid can be used. In the present invention,particularly, at least one selected from the group consisting ofpolyethylene terephthalate, polypropylene terephthalate, polyethyleneisophthalate, polyethylene naphthalate (polyethylene-2,6-naphthalate)and a copolymer thereof is preferred from the viewpoint of mechanicalstrength, thermal resistance, chemical resistance and durability.

Further, the concentration of a COOH end group in the film is preferablyin the range of not less than 15 eq/10⁶ g and not more than 80 eq/10⁶ g,more preferably in the range of not less than 20 eq/10⁶ g and not morethan 50 eq/10⁶ g.

In the polyester, inorganic particles or organic particles, or othervarious additives such as antioxidants, antistatic agents and crystalnuclei agents may be added.

As the material of the inorganic particles, for example, an oxide suchas silicon oxide, aluminum oxide, magnesium oxide, zinc oxide, ironoxide or titanium oxide, a composite oxide such as kaolin, talc ormontmorillonite, a carbonate such as calcium carbonate or bariumcarbonate, a sulfate such as calcium sulfate or barium sulfate, atitanate such as barium titanate or potassium titanate, a phosphate suchas tert calcium phosphate, secondary calcium phosphate or primarycalcium phosphate, a fluoride such as calcium fluoride (fluorite),lithium fluoride or carbon fluoride, and a silicate such as sodiumsilicate or aluminum silicate can be used, but the material is notparticularly restricted by these materials. Further, two or more kindsof particles may be used together depending upon the purpose.

As the organic particles, for example, polystyrene or crosslinkedpolystyrene particles, styrene-acrylic based or acrylic basedcrosslinked particles, vinyl based particles such as styrene-methacrylicbased or methacrylic based crosslinked particles and particles ofbenzoguanamine-formaldehyde, silicone, polytetrafluoroethylene,polyphenylester or phenol can be used, but the particles are notparticularly restricted by these particles. The particles may beparticles at least a part of which is insoluble to polyester.Preferably, a copolymer of a monovinyl compound (A) having only onealiphatic unsaturated bond in the molecule and a compound (B) having twoor more aliphatic unsaturated bonds in the molecule which is used as acrosslinking agent can be employed.

As examples of the compound (A) in the above copolymer, an aromaticmonovinyl compound such as styrene, α-methylstyrene, fluorostyrene orvinyl pyridine, a vinyl cyanide compound such as acrylonitrile ormethacrylonitrile, an acrylate monomer such as methylacrylate,ethylacrylate, propylacrylate, butylacrylate, octylacrylate,dodecylacrylate, hexadecylacrylate, 2-ethylhexylacrylate,2-hydroxyethylacrylate, glycidylacrylate orN,N'-dimethylaminoethylacrylate, a methacrylate monomer such asmethylmethacrylate, ethylmethacrylate, propylmethacrylate,isopropylmethacrylate, butylmethacrylate, sec-butylmethacrylate,arylmethacrylate, phenylmethacrylate, benzylmethacrylate,2-ethylhexylmethacrylate, 2-hydroxyethylmethacrylate,glycidylmethacrylate or N,N'-dimethylaminoethylmethacrylate, a mono- ordicarboxylic acid and an acid anhydride of dicarboxylic acid such asacrylic acid, methacrylic acid, maleic acid or itaconic acid, or anamide based monomer such as acrylic amide or methacrylic amide can beused.

The above-described compound (A) preferably includes the followingstructural formula, and particularly a compound having the number ofcarbon of R₂ of not less than 4 is preferred for providing a flexiblesegment. ##STR1##

In the above formula, R₁ represents H or CH, and R₂ represents H or analkyl group having a number of carbon of not less than 1.

It is particularly preferred that, when the compound (A) is composed ofa single component, the glass transition temperature thereof is nothigher than the glass transition temperature of the polyester used inthe present invention, and the glass transition temperature ispreferably not higher than 50° C., more preferably not higher than 20°C., and further more preferably not higher than 0° C. Concretely,preferably an acrylate monomer such as butylacrylate, octylacrylate,dodecylacrylate, hexadecylacrylate or 2-ethylhexylacrylate and amethacrylate monomer such as butylmethacrylate, sec-butylmethacrylate,hexylmethacrylate, hexadecylmethacrylate or 2-ethylhexylmethacrylate canbe employed.

As examples of the compound (B), a divinylbenzene compound, or apolyfunctional acrylate or methacrylate such as ethylene glycoldiacrylate, ethylene glycol dimethacrylate, polyethylene glycoldiacrylate, polyethylene glycol dimethacrylate, 1,3-butylene glycoldiacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropanetriacrylate or trimethylolpropane trimethacrylate can be employed. Amongthese compounds (B), particularly divinylbenzene, ethylene glycoldimethacrylate or trimethylolpropane trimethacrylate is preferably used.

These compounds (A) and (B) can be used as a mixture of two or morekinds of the respective compounds (A) and (B). Further, The content of apure crosslinking agent in the organic particles is preferably in therange of 1 to 90% by weight, more preferably in the range of 10 to 80%by weight, further more preferably in the range of 20 to 70% by weight.Further, components other than the compounds (A) and (B) may be added,and in order to improve the thermal resistance and the dispersionproperty, coating or surface treatment using a small amount of aninorganic substance may be performed.

As examples of the organic particles having a preferred composition, acrosslinked polymeric particles composed of butylacrylate-divinylbenzenecopolymer, octylacrylate-divinylbenzene copolymer,2-ethylhexylacrylate-divinylbenzene copolymer,2-ethylhexylacrylate-ethylene glycol dimethacrylate copolymer,hexylmethacrylate-divinylbenzene copolymer or2-ethylhexylmethacrylate-divinylbenzene copolymer can be employed.

Particularly, butylacrylate-divinylbenzene copolymer and2-ethylhexylacrylate-divinylbenzene copolymer are preferred. Further,the particles may be composed by a three-component system such asstylene-butylacrylate-divinylbenzene copolymer orstylene-hexylmethacrylate-divinylbenzene copolymer.

Further, the organic particles preferably have a spherical shape and auniform distribution of particle diameter from the viewpoint of goodslipping property and abrasion resistance. Namely, the volume shapefactor thereof is preferably in the range of 0.35 to 0.52, morepreferably in the range of 0.45 to 0.51. (Where, the volume shape factor"f" is calculated by the following equation.

    f=V/D.sup.3

In the equation, "V" represents a volume of a particle (μm³) and "D"represents the maximum diameter of the particle on a plane ofprojection.)

The process for producing the organic particles will be explainedexemplifying a case of crosslinked organic polymeric particles. Forexample, there are processes for producing the particles by thefollowing emulsion polymerizations after mixing compounds (A) and (B).

(a) soap free polymerization process: i.e., process for polymerizingwithout an emulsifier or using a very small amount of an emulsifier

(b) seed polymerization process for adding polymer particles in apolymerization system prior to emulsion polymerization and thereafteremulsion polymerizing

(c) core shell polymerization process for emulsion polymerizing a partof a monomer component and polymerizing the residual monomer in thepolymerization system

(d) polymerization process by the "ugel stat" disclosed in JP-A-SHO54-97582 and JP-A-SHO 54-126288

In the above processes, particularly the processes of (c) and (d) arepreferred to prepare organic particles having a uniform distribution ofparticle diameter.

The particles contained are preferably at least one selected from thegroup consisting of titanium oxide, silicon oxide, aluminum oxide,zirconium oxide, kaolin, talc, calcium phosphate, calcium carbonate,carbon black and organic particles.

Although the particle diameter, content and shape of these particles canbe selected depending upon the use and the purpose, usually the meanparticle diameter is preferably not less than 0.005 μm and not more than3 μm, and the content is preferably not less than 0.01% by weight andnot more than 10% by weight.

Further, the film according to the present invention may be a laminatedfilm having two or more layers. In such a case of a laminated filmhaving two or more layers, the ratio (d/t) of the mean particle diameter(d) of particles contained in at least one layer to the thickness (t) ofthe layer is preferred to be not less than 0.1 and not more than 10.

Where, the mean particle diameter of particles means a mean diameter ofparticles having a potential function for forming protrusions on a filmsurface, and it is determined by observing particles at a number of notless than 100 and not more than 1000 by a transmission type electronmicroscope, determining spherical equivalent diameters thereof andcalculating a cumulative 50% equivalent diameter.

In the present invention, the total thickness of the film can beappropriately decided depending upon the use and the purpose.

Usually, in use for magnetic materials, the thickness is preferably notless than 1 μm and not more than 20 μm, particularly, in use for coatingtype magnetic recording media used for digital video tape recorders, thethickness is preferably not less than 2 μm and not more than 8 μm, andin use for metal evaporated magnetic recording media used for digitalvideo tape recorders, the thickness is preferably not less than 3 μm andnot more than 9 μm.

Further, among uses for industrial materials, in use for heat transferribbons, the thickness is preferably not less than 1 μm and not morethan 6 μm, in use for capacitors, the thickness is preferably not lessthan 0.1 μm and not more than 15 μm, and in use for thermosensiblestencil printing plates, the thickness is preferably not less than 0.1μm and not more than 5 μm.

The film according to the present invention, which has an intensityratio R of not less than 6 determined by laser Raman scattering method,can be preferably used in use for magnetic recording media, use for heattransfer ribbons, use for capacitors and use for thermo-stencil printingplates. In case of magnetic recording media, the F-5 value in thetransverse direction is preferably not less than 12 kg/mm², morepreferably not less than 15 /mm², to obtain a good contact property of atape to a magnetic head and to improve the durability at the time ofhigh-speed running, and such a property is effective for uses for astandard video cassette, an 8 millimeter video cassette, an audiocassette, a floppy disk, etc.

Particularly, in use for digital video tape recorders among uses formagnetic recording media, it is preferred that, besides the intensityratio R determined by laser Raman scattering method is not less than 6,at least one surface is an extremely flat surface having a surfaceroughness (Ra) of 0.1 to 5 nm in order to achieve a high-levelelectromagnetic conversion property.

In order to accomplish this purpose, it is preferred that the film isformed as a laminated film having at least two layers and the aboveproperty can be achieved on at least one surface layer.

In the case of use for heat transfer ribbons, when the above-describedintensity ratio R is not less than 6 and the heat shrinkage of the filmin the transverse direction of the film determined at 100° C. for 30minutes is not more than 3%, preferably not more than 1%, the flatnessof the ribbon at the time of printing can be maintained, irregularity inprinting letters or excessive transfer of ink can be prevented andprinting with a high gradient can be achieved.

In the case of use for capacitors, when the above-described intensityratio R is not less than 6 and the elongation at breakage of the film inthe longitudinal direction of the film is not more than 100%, preferablynot more than 80%, and the elongation at breakage of the film in thetransverse direction of the film is not more than 200%, preferably notmore than 170%, increase of the breakdown voltage and stabilization ofthe dielectric properties can be effectively achieved.

In the case of use for thermo-stencil printing plates, when theabove-described intensity ratio R is not less than 6 and the heat offusion (ΔHu) of the film is not more than 12 cal/g, the punchingproperty at a low energy is excellent, it is possible to change thepunching diameter depending upon an energy level, and the printingproperty is excellent even when a color printing is performed using aplurality of plates.

In the present invention, the film is preferably a biaxially orientedfilm in which a principal orientation axis is present in thelongitudinal direction. In a biaxially oriented film, this means a filmin which the refractive index in the longitudinal direction is greaterthan the refractive index in the transverse direction. Although therespective refractive indexes in the longitudinal direction and in thetransverse direction can be determined by an Abbe refractometer, thedetermination of the direction of the principal orientation axis is alsopossible by a polarization microscope equipped with a Berek compensator.As a biaxially stretching process, a so-called sequential biaxiallystretching process for stretching a cast film in the longitudinaldirection between rollers having a difference between thecircumferential speeds, and thereafter stretching the film in thetransverse direction and heat treating the film in a tenter which holdsthe edges of the film by clips, can be used most appropriately.

Where, "a cast film" means a film formed by supplying sufficiently driedraw material pellets to an extruder, extruding a molten polymer onto arotating metal casting drum in a form of a sheet through a T-die, andcooling and solidifying it, or a film formed by supplying non-driedpellets to a vent-type extruder and forming a sheet similarly. Althoughthe edge portions of a cast film can be formed thicker than the centralportion by a neck down generated at the time of casting, it isinsufficient. Therefore, a desired thickness of the edge portions of acast film can be achieved by increasing the amount of the flow at theedge portions by enlarging a gap between lips of the T-die at positionsof the edge portions or raising the temperature of the edge portions ofthe die. Usually, a thickness profile of a cast film in the transversedirection is a U-shaped profile, and the outermost end portions thereofare formed to be the thickest.

In order to obtain a biaxially oriented polyester film according to thepresent invention which has a peak intensity ratio R (=I_(MD) /I_(ND))determined by laser Raman scattering method of not less than 6, a ratioof the thickness of the edge portions to the thickness of the centralportion of the film at the time of longitudinal stretching iscontrolled. If the ratio of the thickness of the edge portions to thethickness of the central portion of the film at the time of longitudinalstretching is less than 2, irregularity in thickness and irregularity inproperties in the transverse direction become great by snake-moving orvariation of width in the longitudinal stretching·pre-heating process orby neck down during the stretching. If the ratio of the thickness of theedge portions to the thickness of the central portion is over 6, thetemperature of the edge portions cannot be sufficiently raised in thelongitudinal stretching·pre-heating process, and irregularity instretching and frequency of film breakage increase. Therefore, a ratio(A/B) of the maximum thickness (A) of an edge portion of a cast film tothe thickness (B) of a central portion in the transverse direction ofthe cast film is controlled in the range of 2 to 6, preferably in therange of 2.5 to 5.

The longitudinal stretching in the present invention means stretchingfor providing a molecular orientation in the longitudinal direction to afilm. A cast film formed so as to control the ratio of the thickness ofthe edge portion to the thickness of the central portion is firstlyheated at a temperature of 100° to 120° C. by a plurality of heatedrollers, a first-stage longitudinal stretching is performed at a drawratio of 1.5 to 2.5 times between rollers having a circumferential speeddifference, the film is cooled at a temperature of 70° to 98° C. by aplurality of rollers, and a second-stage longitudinal stretching isperformed.

In the present invention, it is preferred that the longitudinalstretching is thus performed by combination of the first-stagestretching at a high temperature and the second-stage stretching at alow temperature. The reason why the temperature of the first-stagelongitudinal stretching is set to a high value is for sufficientlyheating the edge portions which are likely to lack in pre-heating, andalso for enabling a great draw ratio, thereby orienting molecules at ahigh level.

If the temperature is lower than 100° C., these advantages are poor. Ifthe temperature is higher than 120° C., the irregularity in thicknessgreatly deteriorates. Therefore, the temperature is preferably set inthe range of 100° to 120° C. Further, If the draw ratio is less than 1.5times, the advantage for providing a high strength is poor. If the drawratio is more than 2.5 times, the irregularity in thickness becomesgreat. Therefore, the first-stage longitudinal stretching is performedpreferably at a draw ratio of 1.5 to 2.5 times.

The film thus stretched at the first stage is provided with a furtherhigh-level orientation by the successive second-stage longitudinalstretching. In order to stretch the film in the transverse directionwithout problems after the longitudinal stretching, it is effective tostretch the film at a low temperature to suppress the thermalcrystallization in the second-stage longitudinal stretching. Therefore,the second-stage longitudinal stretching is performed preferably at atemperature of not higher than 98° C. However, if the film is stretchedat a temperature lower than 70° C., the stretching becomes a coldstretching, and whitening and breakage of the film frequently occurs.Therefore, the second-stage longitudinal stretching is performedpreferably at a temperature of 70° to 98° C. The draw ratio ispreferably set in the range of 2 to 3.5 times to suppress irregularityin stretching and breakage of the film, although it depends upon thedraw ratio of the first-stage longitudinal stretching.

The temperature process from the first stage to the second stage ispreferably along a monotonic lowering course. By such a temperatureprocess, the insufficient pre-heating of the edge portions heated by thefirst-stage longitudinal stretching can solved, and it becomes possibleto solve the problems with respect to irregularity in properties andbreakage of the film.

The longitudinally stretched film thus obtained is successivelyintroduced into a tenter in which clips grasp both end portions of therunning film, and a transverse stretching and a heat treatment areperformed therein. If the draw ratio in the transverse direction is lessthan 3 times, the properties in the transverse direction is hardlyimproved. If the draw ratio is more than 6 times, breakage of the filmfrequently occurs. Therefore, the draw ratio of the transversestretching is set preferably in the range of 3 to 6 times. Thetemperature for the transverse stretching is set preferably in the rangeof 80° to 200° C., although it depends upon the draw ratio of thetransverse stretching. Particularly, in a case where the film isstretched in the transverse direction at a high draw ratio to increasethe strength in the transverse direction, the so-called temperatureescalation stretching process, (in which the temperature in a stretchingzone of the tenter is stageably elevated for stretching in the range of80° to 200° C.), is preferred from the viewpoint of producing the filmwithout breakage.

In order to provide properties of flatness and thermally dimensionalstability to the biaxially oriented film, a heat treatment at atemperature of 180° to 230° C. is successively performed. Further, inorder to further improve the thermal dimensional stability in thetransverse direction, a so-called relaxation (in which the width of thefilm is contracted at a position from the latter half of the heattreatment zone to the cooling zone in the tenter) is appropriatelyperformed.

Next, examples of the process for producing the biaxially orientedpolyester film according to the present invention will be explained, butthe present invention is not limited by these examples.

Polyethylene terephthalate pellets prepared as the polyester (forexample, pellets added with wet silica particles having a specificsurface area of 300 m² /g at a content of 0.5% by weight, intrinsicviscosity: 0.681, concentration of COOH end group: 42 eq/10⁶ g) aresufficiently dried under a vacuum. The pellets are supplied to anextruder heated at 270° to 300° C., and extruded in a form of a sheetthrough a T-die.

The molten sheet is cast onto a drum cooled at a surface temperature of10° to 40° C. utilizing an electrostatic force, and cooled andsolidified thereon to obtain a substantially amorphous non-stretchedcast film. The cast film is formed so that the ratio (A/B) of themaximum thickness (A) of the edge portions and the thickness (B) of thecentral portion is in the range of 2 to 6 by adjustment of the slit gapof the die in the transverse direction. The cast film is introduced intoa plurality of heated rollers, preheated at a temperature of 100° to120° C., and while the temperature is maintained, the first-stagelongitudinal stretching is performed at a draw ratio of 1.5 to 2.5times. Successively, the film is cooled by a plurality of rollers at atemperature of 70° to 98° C., and after the second-stage longitudinalstretching is performed at that temperature, the film is quickly cooledby rollers having a temperature of 20° to 50° C. Thereafter, the film isintroduced into a tenter grasping both end portion of the film by clips,and the film is stretched in the transverse direction at a draw ratio of3 to 6 times in a hot air atmosphere heated at a temperature of 80° to200° C.

In order to provide flatness and dimensional stability to the film thusbiaxially stretched, the film is heat set in the tenter at a temperatureof 180° to 230° C., and thereafter cooled uniformly and gradually downto a room temperature and wound to obtain an aimed biaxially orientedpolyester film having a peak intensity ratio R (=I_(MD) /I_(ND))determined by laser Raman scattering method of not less than 6.

In the present invention, in order to provide a good slipping propertyto the film, a releasing agent may be provided to one surface of thepolyester film before or after each stretching or during eachstretching.

Although a releasing agent such as a silicone oil, a fluoro-based resinor a surface active agent can be used, particularly the followingreleasing agents are preferred.

Namely, a releasing agent whose main constituent is a mixture of apetroleum wax (C) dissolved, emulsified and suspended in water, a plantsystem wax (D) and an oil substance (E), is particularly preferred.Where, the "main constituent" means that the content by weight of themixture of the above (C), (D) and (E) is not less than 50%, preferablynot less than 60%.

As the petroleum system wax, paraffin wax, microcrystalline wax and waxoxide can be used. Among these waxes, wax oxide is particularlypreferred.

As the plant system wax, candelilla wax, carnauba wax, haze wax,olicurie wax, sugar cane wax, etc. can be used, and in the presentinvention particularly the waxes comprising the following compounds arepreferred.

Namely, rosin, non-uniform rosin, or hydrogenated rosin·αβ-substitutedethylene (α-substitution group: carboxyl, β-substitution group:hydrogen, methyl or carboxyl) added material!·alkyl or alkenyl (carbonnumber: 1-8) poly (repeated unit: 1-6) alcohol added with ester isparticularly preferably used.

The mixing ratio of petroleum system wax to plant system wax ispreferably in the range of 10/90-90/10% by weight, more preferably inthe range of 20/80-80/20% by weight, further more preferably in therange of 30/70-70/30% by weight. The reason why the amount of the plantsystem wax is not less than 10% by weight is to appropriately achieveuniform dispersion when the wax is emulsified or suspended in water andobtain a uniform coating layer. Further, when the amount of thepetroleum system wax is not less than 10% by weight, the slippingproperty of a coating layer is good.

In the present invention, a mixture in which an oil substance is furtheradded to the above-described petroleum wax (C) and plant system wax (D)can be used. Where, the oil substance means an oil which is in acondition of a liquid or a paste at a room temperature, and plant oils,fats and fatty oils, mineral oils and synthesized lubricant oils can beused. As the plant oils, linseed oil, kaya oil, safflower oil, soybeanoil, chinese wood oil, sesame oil, maize oil, rape seed oil, bran oil,cotton-seed oil, olive oil, sasanqua oil, tsubaki oil, caster oil,peanut oil, palm oil and coconut oil can be used. As the fats and fattyoils, beef tallow, pig tallow, sheep tallow and cocoa butter can beused. As the mineral oils, machine oil, insulating oil, turbine oil,motor oil, gear oil, cutting oil and liquid paraffin can be used. As thesynthesized lubricant oils, any of the oils satisfying the requirementsdescribed in the chemical dictionary published by Kyoritsu (a Japanesepublisher) can be used, and for example, olefin polymerized oil, diesteroil, polyalkylene glycol oil and silicone oil can be used. Among theseoils, mineral oils and synthesized lubricant oils are preferred.Further, a mixture of these oils may be used.

The above-described oil substance (E) is preferred to be added at acontent of 1-100 parts by weight, preferably 3-50 parts by weight,relative to 100 parts by weight of a mixture of petroleum wax (C) andplant system wax (D). When such a mixture of a plant system wax, apetroleum wax and an oil substance is used, as compared with a casewhere only one of them is used, a uniform coating layer can be formedmore easily, and the running ability is good, thereby preventingoccurrence of sticking.

In the above-described composition, various additives can be usedtogether as long as the advantages according to the present inventionare not injured. For example, antistatic agent, heat resisting agent,antioxidant, organic and inorganic particles and pigment can be used.

Further, in order to improve the dispersion property in water, variousadditives, (e.g., dispersion assistants, surface active agents,antiseptic agents or antifoaming agents), may be added to the coatingmaterial.

The thickness of the releasing-material layer is preferably not lessthan 0.005 μm and not more than 0.4 μm, more preferably not less than0.01 μm and not more than 0.4 μm.

In the present invention, when the releasing-material layer is coated,the coating material is preferably one dissolved, emulsified orsuspended in water from the viewpoint of explosion-proof property andenvironmental pollution.

The coating of the releasing material may be performed at a stage eitherbefore or after the stretching of the film. In order to achieve theadvantages according to the present invention more remarkably, thecoating before the stretching is particularly preferred. Although themethod for the coating is not particularly restricted, coating using aroll coater, a gravure coater, a reverse coater or a bar coater ispreferred. Further, before coating the releasing material, as needed,corona discharge treatment in an atmosphere of air or other gases may beapplied to the surface to be coated.

Further, the film according to the present invention may be combinedwith other materials such as a paper, a non-woven fabric and a metal.

For example, in a case where a metal is combined, it is possible tolaminate a steel plate or an aluminum plate and the film according tothe present invention to each other and to apply the laminate to use forarchitectural, industrial and can materials.

Further, in a case of use for thermo-stencil printing plates, a poroussupporting material can be laminated on one surface of the filmaccording to the present invention to form an ink path. In this case,although the porous supporting material is not particularly restricted,it must have through holes in the thickness direction, and therefore apaper, a Japanese paper or a fibrous confounding material such as anon-woven fabric is preferred.

Where, although separately produced materials may be laminated by usingan adhesive or applying heat pressing, it is possible to supply afibrous confounding material having an elongation property to the filmformation process according to the present invention and to directlyheat press it onto the film and co-stretch them. In a case ofco-stretching thus performed, the fibers can be in a condition morestretched, and the strength of a thermosensible stencil printing plateobtained can be further increased.

Method for determination of properties!

(1) Orientation of film determined by laser Raman scattering method:

The measuring conditions of laser Raman spectrometry are as follows.

    ______________________________________    Apparatus:   "Ramanor" U-1000 manufactured by                 Jobin Yvon Corporation    Micro Raman: Measuring arrangement; 180° scatterinq                 Sample table; solid    Light source:                 Ar.sup.+  laser, GLG3300 manufactured by                 NEC Corporation                 Wave length; 515 nm    Spectroscope:                 Constitution; 1m Czerny-Turner type                 double monochrometer                 Diffraction grating; Plane holographic,                 1800 g/mm, 110 × 110 mm                 Dispersion; 9.23 cm.sup.-1 /mm                 Counterlight rejection ratio; 10.sup.-14  (20 cm.sup.-1)    Detector:    PM RCA31034 Hamamatsu 943-02    ______________________________________

A film used for determination is wet polished after enclosed bypolymethylmethacrylate, and the measuring section is set in a directionparallel to the longitudinal direction of the film. The measuringportion is a center portion, the measurement is repeated ten times whilethe measuring position is shifted, and a mean value thereof isdetermined. In the determination, an intensity (I_(MD)) of 1615 cm⁻¹band in the measurement of a polarized light parallel to thelongitudinal direction and an intensity (I_(ND)) of 1615 cm⁻¹ band inthe measurement of a polarized light parallel to the thickness directionare measured, and the ratio R indicating the orientation is representedas R=I_(MD) /I_(ND).

(2) Amorphous orientation coefficient "f_(MD) ":

A total orientation coefficient in laser Raman scattering methodaforementioned is defined by the following equation.

    F.sub.t,MD =(I.sub.MD /I.sub.ND)/(3 F.sub.t,0)

    F.sub.t,0 =(I.sub.MD /I.sub.ND +I.sub.TD /I.sub.ND +I.sub.ND /I.sub.ND)/3

Where, the standard is set as I_(ND) =1. The I_(TD) is an intensity of1615 cm⁻¹ band in the measurement of a polarized light parallel to thetransverse direction in laser Raman scattering method.

Next, an orientation function F_(C),MD of (105) plane is calculated byX-ray pole figure method, and the amorphous orientation coefficientf_(MD) is determined by the following equation.

    f.sub.MD =(F.sub.t,MD -χ·F.sub.C,MD)/(1-χ)

(Where, χ is a crystallization degree of a film determined by densitymethod.)

(3) Inherent viscosity:

Using o-chlorophenol as a solvent, it is determined at 25° C.

(4) Concentration of COOH end group:

A film of 0.5 g is dissolved in o-cresol, and it is titrated bypotassium hydroxide.

(5) F-5 value:

Using an "Instron" type tensile tester, a sample film is tensed at awidth of 10 mm, a distance between clips of 100 mm and a tensile speedof 200 mm/min. In the tension-strain curve obtained, a tension at aposition of 5% elongation is defined as the F-5 value. The determinationis performed in an atmosphere having a temperature of 25° C. and ahumidity of 65% RH.

(6) Refractive index:

Using an Abbe refractometer manufacture by Atago Corporation, therefractive index of a film relative to a sodium D-ray is determined inthe longitudinal direction and transverse direction at 25° C.

(7) Irregularity in thickness:

Using a film thickness tester KG601A and an electronic micrometer K306Cmanufactured by Anritsu Corporation, a film sampled at a width of 30 mmand a length of 10 m in the longitudinal direction is passed through andthe thickness is continuously determined. From the maximum thicknessTmax (μm) and the minimum thickness Tmin (μm) in the length of 10 m,

    R(μm)=Tmax-Tmin

is calculated, and the irregularity in thickness is determined by thefollowing equation using a mean thickness Tave (μm) in the length of 10m.

Irregularity in thickness (%)=R/Tave

(8) Irregularity in birefringence:

Using a Berek compensator for a polarization microscope, a retardationis determined, and the birefringence (Δn) is determined by the followingequation.

    Δn=r/d

Where, r: retardation, d: film thickness.

With respect to the irregularity in birefringence, Δn is determined inthe transverse direction of the film, and the irregularity is determinedby the following standards.

⊚: The difference between the maximum value and the minimum value isless than 0.01.

∘: The difference between the maximum value and the minimum value is notless than 0.01 and less than 0.02.

Δ: The difference between the maximum value and the minimum value is notless than 0.02 and less than 0.03.

×: The difference between the maximum value and the minimum value is notless than 0.03.

(9) Frequency of film breakage:

Polyethylene terephthalate vacuum dried is cast from a T-die onto acasting drum applying an electrostatic force, it is cooled andsolidified to form a cast film, and the cast film is biaxially orientedby a longitudinal stretching apparatus having a plurality of rollers anda tenter and heat treated in the tenter. The film is determined in theprocess by the following standards.

⊚: No breakage from edge portions occurs.

∘: Breakage from edge portions occurs very few times.

Δ: Breakage from edge portions occurs sometimes.

×: Breakage from edge portions occurs frequently.

(10) Electromagnetic conversion property (C/N):

A magnetic coating material and a non-magnetic coating material havingthe following compositions are coated in order by an extrusion coater onthe surface of the film according to the present invention (the upperlayer is composed of the magnetic coating material with a coatingthickness of 0.1 μm, and the thickness of the lower layer composed ofthe non-magnetic coating material is variously changed), the coatedlayer is magnetically oriented, and then it is dried. After forming aback-coat layer having the following composition on the oppositesurface, the film calendered by a small test calendering machine (steelroll/steel roll; 5 stages) at a temperature of 85° C. and a linearpressure of 200 kg/cm, and thereafter, the film is cured at 60° C. for48 hours. The film obtained is slit to make a pancake of a tape having awidth of 8 mm. Then, the tape of 250 m from the pancake is incorporatedinto a cassette to make a cassette tape.

The C/N (carrier/noise ratio) of 7 MHz+1 MHz of the tape is determinedusing a Hi8 VTR (manufactured by SONY Corporation, EV-BS3000) sold onthe market.

    ______________________________________    (All parts are by weight.)    ______________________________________    (Composition of magnetic coating material)    Ferromagnetic metal powder:                              100    parts    Sulfonic Na modified vinyl chloride copolymer:                              10     parts    Sulfonic Na modified polyurethane:                              10     parts    Polyisocyanate:           5      parts    Stearic acid:             1.5    parts    Oleic acid:               1      part    Carbon black:             1      part    Alumina:                  10     parts    Methylethylketone:        75     parts    Cyclohexanone:            75     parts    Toluene:                  75     parts    (Composition of non-magnetic coating material for lower layer)    Titanium oxide:           100    parts    Carbon black:             10     parts    Sulfonic Na modified vinyl chloride copolymer:                              10     parts    Sulfonic Na modified polyurethane:                              10     parts    Methylethylketone:        30     parts    Methylisobutylketone:     30     parts    Toluene:                  30     parts    (Composition of back coat)    Carbon black (mean diameter: 20 nm):                              95     parts    Carbon black (mean diameter: 280 nm):                              10     parts    α-Alumina:          0.1    part    Zinc oxide                0.3    part    Sulfonic Na modified polyurethane:                              20     parts    Sulfonic Na modified vinyl chloride copolymer:                              30     parts    Cyclohexanone:            200    parts    Methylethylketone:        300    parts    Toluene:                  100    parts    ______________________________________

(11) Irregularity in printing and Gradient:

Ink layers of cyanogen, fuchsine and yellow are coated on the obtainedfilm to form an ink ribbon, a standard printing of color pattern isperformed by a variable dot type heat transfer color printer, and thegradient is determined by observation. Further, whether wrinkles aregenerated in the ribbon or not is determined together by observinguniformity of printed portions.

(12) Properties for capacitors:

Dielectric property

A sample is prepared by depositing an aluminum on both surfaces of afilm in a circular form having a diameter of 18 mm so that the thicknessthereof is in the range of 600 to 1000 Å, and the sample is left in anatmosphere having a temperature of 20°±5° C. and a relative humidity of65±5% for a time of not less than 48 hours.

Using a dielectric property measuring apparatus DEA-2970 manufactured byTA instruments Corporation, the temperature dependency of dielectricloss is determined at a frequency of 1 kHz and a temperature elevationspeed of 2° C./min., and a case where the dielectric loss determined at105° C. is not more than 1.3% is determined to be acceptable.

B. Breakdown voltage

It is determined as follows based on the method described in JIS-C-2319but using a film, which is not applied with metal deposition, as asample.

As shown in FIG. 1, a rubber plate 2 having a rubber Shore hardness ofabout 60 degrees and a thickness of about 2 mm is placed on a metalplate 1 having an appropriate size, ten aluminum foils 3 each having athickness of about 6 μm are stacked thereon to form a lower electrode,and a brass column 4 having a weight of about 50 g, an edge with aroundness of about 1 mm and a flat and scratch-less bottom surface witha diameter of 8 mm is prepared as an upper electrode. The sample 5 isleft in an atmosphere having a temperature of 20°±5° C. and a relativehumidity of 65±5% for a time of not less than 48 hours in advance. Thesample 5 is nipped between the upper electrode 4 and the lower electrode3, a DC voltage from a DC power source 6 is applied between bothelectrodes by the circuit shown in FIG. 1 in an atmosphere having atemperature of 20°±5° C. and a relative humidity of 65±5%, and the DCvoltage is raised from 0 V to a voltage causing a breakdown at a speedof 100 V/sec. The test is performed on 50 samples, a mean value ofvalues each calculated by dividing the measured breakdown voltage by thethickness of the sample is determined, and a case where the mean valueis not less than 400 V/μm is determined to be acceptable.

EXAMPLES

More concrete examples of the present invention will be hereunderexplained.

Examples 1-7

After pellets of polyethylene terephthalate (inherent viscosity: 0.65,glass transition temperature: 69° C., melting point: 256° C.,concentration of COOH end group: 36 eq/10⁶ g, added with calciumcarbonate particles having a mean diameter of 0.23 μm at a content of0.03% by weight) were vacuum dried at 180° C. for 3 hours, they weresupplied to an extruder heated at 280° C. and melt-extruded therefrom,and the molten polymer was delivered out from a T-die in a form of asheet. The sheet was cast on a cooling drum having a temperature of 25°C. while applied with an electrostatic force, and it was cooled andsolidified to prepare a non-stretched cast film. The thickness of thecentral portion of the cast film was all adjusted to 150 μm, and thecast film was formed so that the ratio (A/B) of the maximum thickness ofthe edge portion (A) to the thickness of the central portion (B) was inthe range of 2 to 6.

The non-stretched film was introduced into a plurality of heated rollersand pre-heated at 110° C., and then, the film was stretched at a drawratio of 2 times in a first-stage longitudinal stretching process. Afterthe film was cooled at 80° C., the film was stretched at a draw ratio of3 times in a second-stage longitudinal stretching process. The film wasintroduced into a tenter which grasps both end portions of the film byclips, and therein the film was stretched in the transverse direction ata temperature of 90° C. and a draw ratio of 3.5 times, and thereafterheat treated at 200° C. The peak intensity ratio R determined by laserRaman scattering method, the amorphous orientation coefficient f_(MD),the F-5 value, the irregularity in thickness, the irregularity in doublerefraction and the frequency of film breakage of each film obtained areshown in Tables 1 and 2. The concentration of COOH end group in the filmwas 42 eq/10⁶ g.

Comparative Examples 1-4

In Comparative Examples 1-4, only the maximum thickness of the edgeportions of the cast film was changed, and biaxially oriented films wereproduced at the same film formation conditions. The peak intensity ratioR determined by laser Raman scattering method, the amorphous orientationcoefficient f_(MD), the F-5 value, the irregularity in thickness, theirregularity in double refraction and the frequency of film breakage ofeach film obtained are shown in Tables 1 and 2.

Examples 8-14

In Examples 8-14, the cast film was formed so that the thickness of thecentral portion was 150 μm and the maximum thickness of the edgeportions was 400 μm. After the non-stretched film was pre-heated byintroducing it into a plurality of heated rollers, the first-stagelongitudinal stretching was performed, the film was then cooled by aplurality of rollers, and thereafter the second-stage longitudinalstretching was performed. The film was introduced into the tentergrasping the end portions of the film by clips, and after stretched inthe transverse direction at a temperature of 90° C. and a draw ratio of3.5 times, the film was heat treated at 200° C. The peak intensity ratioR determined by laser Raman scattering method, the amorphous orientationcoefficient f_(MD), the F-5 value, the irregularity in thickness, theirregularity in double refraction and the frequency of film breakage ofeach film obtained are shown in Tables 3 and 4.

Example 16, Comparative Examples 5-9

After pellets of polyethylene terephthalate (inherent viscosity: 0.65,glass transition temperature: 69° C., melting point: 256° C.) werevacuum dried at 180° C. for 3 hours, they were supplied to an extruderheated at 280° C., and the molten polymer was delivered out from a T-diein a form of a sheet. The sheet was cast on a cooling drum having atemperature of 25° C. while applied with an electrostatic force, and itwas cooled and solidified to prepare a non-stretched cast film. Thethickness of the central portion of the cast film was all adjusted to150 μm, and each cast film was formed by changing the ratio (A/B) of themaximum thickness of the edge portion (A) to the thickness of thecentral portion (B).

The non-stretched film was stretched in the longitudinal direction at atemperature of 100° C. and a draw ratio of 3.2 times, and the film wasintroduced into the tenter grasping both end portions of the film byclips and stretched in the transverse direction at a temperature of 90°C. and a draw ratio of 3.5 times. Then, the biaxially oriented film wasre-stretched in the longitudinal direction at a temperature of 140° C.and a draw ratio of 1.6 times, and thereafter the film was heat treatedat 200° C. Where, the longitudinal stretching was performed betweenrollers having different circumferential speeds and the transversestretching and the heat treatment were performed in the tenter graspingboth end portions of the film by clips. The peak intensity ratio Rdetermined by laser Raman scattering method, the amorphous orientationcoefficient f_(MD), the F-5 value, the irregularity in thickness, theirregularity in birefringence and the frequency of film breakage of eachfilm obtained are shown in Table 5.

Examples 17-20

Using two extruders A and B, pellets of polyethylene terephthalate X(inherent viscosity: 0.661, glass transition temperature: 69° C.,melting point: 256° C., concentration of COOH end group: 33 eq/10⁶ g,added with spherical silica particles having a mean diameter of 100 nmat a content of 0.02% by weight) were supplied to the extruder A heatedat 280° C. after vacuum dried at 180° C. for 3 hours, and pellets ofpolyethylene terephthalate Y (inherent viscosity: 0.675, glasstransition temperature: 69° C., melting point: 255° C., concentration ofCOOH end group: 43 eq/10⁶ g, added with spherical divinylbenzene-styrenecopolymer particles (copolymerization mole ratio: 60: 40) having a meandiameter of 180 nm at a content of 0.05% by weight) were supplied to theextruder B heated at 280° C. after vacuum dried at 180° C. for 3 hours.The molten polymers were jointed in a T-die and the polymer sheet wascast on a cooling drum having a temperature of 30° C. while applied withan electrostatic force, and it was cooled and solidified to prepare anon-stretched laminated cast film.

Biaxially oriented laminated films were prepared in a manner similar tothat of Example 4, changing the lamination thickness ratio of the film.The peak intensity ratio R determined by laser Raman scattering method,the amorphous orientation coefficient f_(MD), the F-5 value, theirregularity in thickness, the irregularity in birefringence and thefrequency of film breakage of each film obtained are shown in Table 6.The concentration of COOH end group in the film was 48 eq/10⁶ g.

Example 21

Using two extruders A and B, pellets of polyethylene terephthalate X(inherent viscosity: 0.660, glass transition temperature: 69° C.,melting point: 256° C., concentration of COOH end group: 35 eq/10⁶ g,added with spherical silica particles having a mean diameter of 40 nm ata content of 0.4% by weight) were supplied to the extruder A heated at280° C. after vacuum dried at 180° C. for 3 hours, and pellets ofpolyethylene terephthalate Y (inherent viscosity: 0.643, glasstransition temperature: 69° C., melting point: 255° C., concentration ofCOOH end group: 40 eq/10⁶ g, added with spherical divinylbenzene-styrenecopolymer particles (copolymerization mole ratio: 60: 40) having a meandiameter of 180 nm at a content of 1.0% by weight) were supplied to theextruder B heated at 280° C. after vacuum dried at 180° C. for 3 hours.The molten polymers were jointed in a T-die (lamination ratio X/Y=30/1)and the polymer sheet was cast on a cooling drum having a temperature of30° C. while applied with an electrostatic force, and it was cooled andsolidified to prepare a non-stretched laminated cast film. The ratio(A/B) of the maximum thickness of the edge portion (A) to the thicknessof the central portion (B) of the film was 4.5.

The film was introduced into a plurality of heated rollers andpre-heated at 110° C., and then, the film was stretched at a draw ratioof 2.2 times in the first-stage longitudinal stretching process, andafter cooled by a plurality of rollers at 80° C., the film was stretchedat a draw ratio of 2.6 times in a second-stage longitudinal stretchingprocess. The film was introduced into the tenter grasping both endportions of the film by clips, and therein the film was stretched in thetransverse direction at a temperature of 90° C. and a draw ratio of 3.6times, and thereafter heat treated at 120° C.

Then, the film was stretched in the transverse direction at a draw ratioof 1.6 times and heat treated at 210° C. to prepare a biaxially orientedpolyester film. The total thickness was 6.2 μm (X/Y=6.0 μm/0.2 μm), andthe concentration of COOH end group in the film was 41 eq/10⁶ g. Thepeak intensity ratio R determined by laser Raman scattering method, theamorphous orientation coefficient f_(MD), the F-5 value and theelectromagnetic conversion property of the film obtained are shown inTable 7, and it was a very good film for use for magnetic recordingmedia.

Examples 22-24

Biaxially oriented polyester films were prepared in a manner similar tothat of Example 21 other than conditions changing draw ratio andtemperature in stretching. The resulted films were good for use formagnetic recording media as shown in Table 7.

Example 25

After pellets of polyethylene terephthalate (inherent viscosity: 0.626,glass transition temperature: 69° C., melting point: 256° C.,concentration of COOH end group: 45 eq/10⁶ g, added with agglomeratedsilica particles having a mean diameter of 0.23 μm at a content of 0.06%by weight) were vacuum dried at 180° C. for 3 hours, they were suppliedto an extruder heated at 280° C. and the molten polymer was deliveredout from a T-die in a form of a sheet. The sheet was cast on a coolingdrum having a temperature of 25° C. while applied with an electrostaticforce, and it was cooled and solidified to prepare a non-stretched castfilm. The thickness of the central portion of the cast film was alladjusted to 150 μm, and the cast film was formed so that the ratio (A/B)of the maximum thickness of the edge portion (A) to the thickness of thecentral portion (B) was 3.5.

The non-stretched film was introduced into a plurality of heated rollersand pre-heated at 110° C., and then, the film was stretched at a drawratio of 2.2 times in a first-stage longitudinal stretching process.After the film was cooled at 80° C., the film was stretched at a drawratio of 3.0 times in a second-stage longitudinal stretching process.The film was introduced into the tenter grasping both end portions ofthe film by clips, and therein the film was stretched in the transversedirection at a temperature of 90° C. and a draw ratio of 4.2 times, andthereafter heat treated at 200° C. and relaxed in the transversedirection at a rate of 3% and a temperature of 120° C. to prepare abiaxially oriented polyester film. The concentration of COOH end groupin the film was 52 eq/10⁶ g. The peak intensity ratio R determined bylaser Raman scattering method, the amorphous orientation coefficientf_(MD) and the color printing property of the film obtained are shown inTable 8. As shown in Table 8, the film was very good as a film for heattransfer ribbons.

Examples 26 and 27

Biaxially oriented polyester films were prepared in the same manner asthat of Example 25 other than conditions changing relaxation. Theresulted films were good as films for heat transfer ribbons as shown inTable 8.

Example 28

After pellets of polyethylene terephthalate (inherent viscosity: 0.65,glass transition temperature: 69° C., melting point: 256° C.,concentration of COOH end group: 27 eq/10⁶ g, added with calciumphosphate particles having a mean diameter of 0.18 μm at a content of0.03% by weight) were vacuum dried at 180° C. for 3 hours, they weresupplied to an extruder heated at 280° C. and the molten polymer wasdelivered out from a T-die in a form of a sheet. The sheet was cast on acooling drum having a temperature of 25° C. while applied with anelectrostatic force, and it was cooled and solidified to prepare anon-stretched cast film. The thickness of the central portion of thecast film was all adjusted to 150 μm, and the cast film was formed sothat the ratio (A/B) of the maximum thickness of the edge portion (A) tothe thickness of the central portion (B) was 4.5.

The non-stretched film was introduced into a plurality of heated rollersand pre-heated at 110° C., and then, the film was stretched at a drawratio of 2.2 times in a first-stage longitudinal stretching process.After the film was cooled at 85° C., the film was stretched at a drawratio of 2.6 times in a second-stage longitudinal stretching process.The film was introduced into the tenter grasping both end portions ofthe film by clips, and therein the film was stretched in the transversedirection at a temperature of 95° C. and a draw ratio of 3.5 times, andthereafter the film was re-stretched in the longitudinal direction at atemperature of 140° C. and a draw ratio of 1.5 times. After the film wasre-stretched in the transverse direction at a temperature of 190° C. anda draw ratio of 1.3 times, the film was heat treated at 200° C. andrelaxed in the longitudinal direction at a temperature of 140° C. and arate of 1% to prepare a biaxially oriented polyester film having athickness of 3 μm. The concentration of COOH end group in the film was33 eq/10⁶ g. The peak intensity ratio R determined by laser Ramanscattering method, the amorphous orientation coefficient f_(MD), thetensile elongations at break in the longitudinal and transversedirections and the properties for capacitors of the film obtained areshown in Table 9. As understood from Table 9, the film was very good asa film for capacitors.

Examples 29 and 30

Biaxially oriented polyester films were prepared in the same manner asthat of Example 28 other than conditions changing draw ratios in thelongitudinal and transverse directions. The resulting films were goodfilms for capacitors as shown in Table 9.

Example 31

After pellets of polyethylene terephthalate-polyethylene isophthalatecopolymer (concentration of COOH end group: 46 eq/10⁶ g, inherentviscosity: 0.702, glass transition temperature: 67° C., melting point:255° C., copolymerization ratio: 80/20, added with agglomerated silicaparticles having a mean diameter of 0.32 μm at a content of 0.2% byweight) were vacuum dried at 120° C. for 3 hours and pre-crystallized,they were vacuum dried at 180° C. for 3 hours, and then they weresupplied to an extruder heated at 270° C. and the molten polymer wasdelivered out from a T-die in a form of a sheet. The sheet was cast on acooling drum having a temperature of 25° C. while applied with anelectrostatic force, and it was cooled and solidified to prepare anon-stretched cast film. The thickness of the central portion of thecast film was all adjusted to 150 μm, and the cast film was formed sothat the ratio (A/B) of the maximum thickness of the edge portion (A) tothe thickness of the central portion (B) was 5.

The non-stretched film was introduced into a plurality of heated rollersand pre-heated at 105° C., and then, the film was stretched at a drawratio of 2.0 times in a first-stage longitudinal stretching process.After the film was cooled at 80° C., the film was stretched at a drawratio of 3.0 times in a second-stage longitudinal stretching process.The film was introduced into the tenter grasping both end portions ofthe film by clips, and therein the film was stretched in the transversedirection at a temperature of 90° C. and a draw ratio of 3.5 times, andthereafter the film was heat treated at 120° C. The thickness of theobtained film was 1.6 μm, the concentration of COOH end group in thefilm was 53 eq/10⁶ g, and the heat of fusion ΔHu was 6 cal/g. The peakintensity ratio R determined by laser Raman scattering method was 7.2and the amorphous orientation coefficient f_(MD) was 0.562. The film waslaminated on a Japanese paper having a weight of 12 g/m² to make athermo-stencil printing plate. As the result of a test pattern printingperformed by "Risograph" manufactured by Riso Kagaku Corporation usingthis plate, the gradient and the printing property were both excellent.

Example 32

A polyethylene terephthalate non-woven fabric having a weight of 300g/m² was laminated on the non-stretched film and the laminate wasintroduced into a plurality of heated rollers.

A biaxially oriented polyester film (a film laminated with a stretchednon-woven fabric on one surface by heat pressing) was prepared in amanner similar to that of Example 31 other than the above-describedcondition. The properties with respect to the film were substantiallythe same as those in Example 31. Lamination with a Japanese paper is notrequired for this composite, and the properties for a thermo-stencilprinting plate were not poor as compared with those of the laminateprepared in Example 31.

Comparative Example 10

Polyester (Y) used in Example 17 was supplied to an extruder heated at280° C. and the molten polymer was delivered out from a T-die in a formof a sheet. The sheet was cast on a cooling drum having a temperature of25° C. while applied with an electrostatic force, and it was cooled andsolidified to prepare a non-stretched cast film. The thickness of thecentral portion (B) of the cast film was 150 μm, and the maximumthickness of the edge portion (A) was 1050 μm (A/B=7).

The non-stretched film was stretched in the longitudinal direction at atemperature of 100° C. and a draw ratio of 3.3 times, and the film wasintroduced into the tenter grasping both end portions of the film byclips, and therein the film was stretched in the transverse direction ata temperature of 95° C. and a draw ratio of 3.5 times, and thereafterthe film was heat treated at 200° C. When the properties of thebiaxially oriented film obtained were determined, the peak intensityratio R (=I_(MD) /I_(ND)) determined by laser Raman scattering methodwas 4.1 and the amorphous orientation coefficient f_(MD) was 0.352, andthe irregularity in thickness was not less than 9% and it was poor.

                                      TABLE 1    __________________________________________________________________________    Cast film                  Conditions of longitudinal stretching    Maximum thickness                   Thickness of                               First stage                                          Second stage    of edge portion (A)                   central portion (B)                               Temperature                                          Temperature    (μm)        (μm)  A/B                               (°C.)                                     Draw ratio                                          (°C.)                                                Draw ratio    __________________________________________________________________________    Example 1          300      150      2.0                               110   2.0  80    3.0    Example 2          375      150      2.5                               110   2.0  80    3.0    Example 3          450      150      3.0                               110   2.0  80    3.0    Example 4          525      150      3.5                               110   2.0  80    3.0    Example 5          600      150      4.0                               110   2.0  80    3.0    Example 6          750      150      5.0                               110   2.0  80    3.0    Example 7          900      150      6.0                               110   2.0  80    3.0    Comparative          270      150      1.8                               110   2.0  80    3.0    Example 1    Comparative          225      150      1.5                               110   2.0  80    3.0    Example 2    Comparative          1050     150      7.0                               110   2.0  80    3.0    Example 3    Comparative          1200     150      8.0                               110   2.0  80    3.0    Example 4    __________________________________________________________________________

                                      TABLE 2    __________________________________________________________________________                                            Peak intensity    F-5 value                               ratio R in                                                   Amorphous    (kg/mm.sup.2)    Principal                           Irregularity                                 Irregularity                                       Frequency                                            laser Raman                                                   orientation    Longitudinal                Transverse                     orientation                           in thickness                                 in birefrin-                                       of film                                            scattering                                                   coefficient    direction   direction                     axis  (%)   gence breakage                                            (=I.sub.MD /I.sub.ND)                                                   f.sub.MD    __________________________________________________________________________    Example 1          19.0  10.8 Longitudinal                           3.5   ◯                                       ◯                                            10.9   0.772                     direction    Example 2          19.0  11.0 Longitudinal                           3.0   ⊚                                       ⊚                                            12.3   0.791                     direction    Example 3          19.5  11.0 Longitudinal                           2.7   ⊚                                       ⊚                                            13.1   0.821                     direction    Example 4          20.0  11.0 Longitudinal                           2.5   ⊚                                       ⊚                                            15.2   0.850                     direction    Example 5          20.0  11.0 Longitudinal                           2.5   ⊚                                       ⊚                                            14.8   0.843                     direction    Example 6          19.5  10.9 Longitudinal                           2.9   ⊚                                       ⊚                                            13.4   0.816                     direction    Example 7          19.0  10.8 Longitudinal                           3.7   ◯                                       ◯                                            12.6   0.783                     direction    Comparative          18.8  10.5 Longitudinal                           5.0   Δ                                       Δ                                            5.2    0.481    Example 1        direction    Comparative          18.5  10.4 Longitudinal                           6.5   X     X    5.1    0.473    Example 2        direction    Comparative          18.7  10.5 Longitudinal                           5.5   Δ                                       Δ                                            5.0    0.468    Example 3        direction    Comparative          18.3  10.2 Longitudinal                           6.9   X     X    5.4    0.475    Example 4        direction    __________________________________________________________________________

                                      TABLE 3    __________________________________________________________________________    Cast film                        Conditions of longitudinal stretching    Maximum thickness                   Thickness of                               Principal                                     First stage                                                Second stage    of edge portion (A)                   central portion (B)                               orientation                                     Temperature                                                Temperature    (μm)        (μm)  A/B                               axis  (°C.)                                           Draw ratio                                                (°C.)                                                      Draw ratio    __________________________________________________________________________    Example 8          400      150      4.0                               Longitudinal                                     105   2.0  80    3.0                               direction    Example 9          400      150      4.0                               Longitudinal                                     115   2.0  80    3.0                               direction    Example 10          400      150      4.0                               Longitudinal                                     110   1.7  80    3.0                               direction    Example 11          400      150      4.0                               Longitudinal                                     110   2.3  80    3.0                               direction    Example 12          400      150      4.0                               Longitudinal                                     110   2.0  75    3.0                               direction    Example 13          400      150      4.0                               Longitudinal                                     110   2.0  90    3.0                               direction    Example 14          400      150      4.0                               Longitudinal                                     110   2.0  95    3.0                               direction    __________________________________________________________________________

                                      TABLE 4    __________________________________________________________________________                                      Peak intensity    F-5 value                         ratio R in                                             Amorphous    (kg/mm.sup.2)    Irregularity                           Irregularity                                 Frequency                                      laser Raman                                             orientation    Longitudinal                Transverse                     in thickness                           in birefrin-                                 of film                                      scattering                                             coefficient    direction   direction                     (%)   gence breakage                                      (=I.sub.MD /I.sub.ND)                                             f.sub.MD    __________________________________________________________________________    Example 8          19.5  11.0 2.3   ⊚                                 ◯                                      11.6   0.755    Example 9          18.8  11.0 3.0   ⊚                                 ⊚                                      9.2    0.700    Example 10          18.5  10.8 2.3   ⊚                                 ⊚                                      9.2    0.708    Example 11          20.2  10.6 3.8   ⊚                                 ⊚                                      15.5   0.831    Example 12          20.3  11.0 2.4   ⊚                                 ◯                                      15.3   0.822    Example 13          19.3  10.7 3.2   ⊚                                 ⊚                                      12.2   0.766    Example 14          18.1  10.5 3.8   ◯                                 ⊚                                      8.9    0.693    __________________________________________________________________________

                                      TABLE 5    __________________________________________________________________________                                                         Peak  Amor-    Cast film                                            intensity                                                               phous    Maximum                                              ratio R                                                               orienta-    thickness   Thickness      F-5 value  Irregu-                                               Irregu-   laser tion    of edge     of central                         Principal                               (kg/mm.sup.2)                                          larity in                                               larity                                                    Frequency                                                         Raman coef-    portion (A) portion (B)                         orientation                               Longitudinal                                     Transverse                                          thickness                                               in birefrin-                                                    of film                                                         scattering                                                               ficient    (μm)     (μm)                      A/B                         axis  direction                                     direction                                          (%)  gence                                                    breakage                                                         (=I.sub.MD /I.sub.ND)                                                         1     f.sub.MD    __________________________________________________________________________    Comparative           225  150   1.5                         Longitudinal                               19.5  9.0  broken in a tener for heat                                          treatment      --    --    Example 5            direction    Comparative          1200  150   8.0                         Longitudinal                               18.3  10.2 3.8  Δ                                                    Δ                                                         5.3   0.452    Example 6            direction    Comparative          1500  150   10.0                         Longitudinal                               18.5  10.4 3.0  Δ                                                    Δ                                                         5.2   0.443    Example 7            direction    Comparative          1800  150   12.0                         Longitudinal                               18.2  10.5 2.5  X    X    5.4   0.431    Example 8            direction    Comparative          2250  150   15.0                         Longitudinal                               17.5  10.7 broken in longitudinal                                          re-stretching  --    --    Example 9            direction    Example 16           750  150   5.0                         Longitudinal                               14.8  12.5 3.6  ◯                                                    ◯                                                         6.5   0.606    __________________________________________________________________________

                                      TABLE 6    __________________________________________________________________________    Thickness  Ratio of                    Ratio of                             Peak    ratio of   particle                    particle                             intensity    polyethylene               diameter                    diameter                             ratio R    terephthalate               to   to   Total                           in                                                              Amor-    (X) to     thickness                    thickness                         thickness                              F-5 value  Irregular-      Raman                                                              phous    polyethylene               of X-layer                    of Y-layer                         of final                              (kg/mm.sup.2)                                         ity in                                              Irregular-                                                    Frequency                                                         scattering                                                              orientation    terephthalate               side side film Longitudinal                                    Transverse                                         thickness                                              ity in                                                    of film                                                         (= I.sub.MD /                                                              coefficient    (Y) (X/Y)  (d/t)                    (d/t)                         (μm)                              direction                                    direction                                         (%)  birefrigence                                                    breakage                                                         I.sub.ND)                                                              f.sub.MD    __________________________________________________________________________    Example         2.0/5.0               0.05 0.16 7    18.8  11.2 2.6  ⊚                                                    ⊚                                                         14.8 0.840    17    Example         1.0/6.0               0.10 0.13 7    19.2  10.6 2.4  ⊚                                                    ⊚                                                         15.1 0.836    18    Example         2.0/4.0               0.05 0.20 6    19.0  10.7 2.4  ⊚                                                    ∘                                                         15.4 0.843    19    Example         0.1/4.9               1.0  0.16 5    19.1  10.7 2.2  ⊚                                                    ⊚                                                         16.0 0.861    20    __________________________________________________________________________

                                      TABLE 7    __________________________________________________________________________    Conditions of first                    Conditions of second                              Conditions of                                        Conditions of    longitudinal stretching                    longitudinal stretching                              transverse stretching                                        transverse re-stretching          Temperature                Draw                    Temperature                          Draw                              Temperature                                    Draw                                        Temperature                                              Draw          (°C.)                ratio                    (°C.)                          ratio                              (°C.)                                    ratio                                        (°C.)                                              ratio    __________________________________________________________________________    Example 21          110   2.2 80    2.6 90    3.6 200   1.6    Example 22          110   2.2 80    2.7 95    3.6 200   1.5    Example 23          110   2.0 80    2.8 95    3.5 200   1.7    Example 24          110   2.2 80    2.8 90    3.2 200   1.8    __________________________________________________________________________                                            Electro-    Surface roughness                   F-5 value  Peak intensity                                      Amorphous                                            magnetic    (nm)           (kg/mm.sup.2)                              ratio R in laser                                      orientation                                            conversion          X-layer               Y-layer                   Longitudinal                         Transverse                              Raman scattering                                      coefficient                                            property          side side                   direction                         direction                              (= I.sub.MD /I.sub.ND)                                      f.sub.MD                                            (C/N)    __________________________________________________________________________    Example 21          0.50 7.2 17.5  19.3 13.2    0.803 +1.2    Example 22          0.63 7.3 19.3  17.5 14.3    0.816 +1.3    Example 23          0.65 7.0 17.8  20.5 13.0    0.791 +1.0    Example 24          0.55 7.3 17.0  21.5 13.4    0.790 +1.5    __________________________________________________________________________

                                      TABLE 8    __________________________________________________________________________               F-5 value             Peak intensity                                             Amorphous               (kg/mm.sup.2)         ratio R in laser                                             orientation    Relaxation Longitudinal                     Transverse                          Heat shrinkage                                     Raman scattering                                             coefficient                                                   Printing property    (%)        direction                     direction                          (100° C. × 30 min., %)                                     (= I.sub.MD /I.sub.ND)                                             f.sub.MD                                                   gradient                                                       wrinkle    __________________________________________________________________________    Example 25          3    18.8  17.3 0.4        13.6    0.760 good                                                       none    Example 26          1    18.9  18.8 0.9        13.7    0.772 good                                                       none    Example 27          5    18.8  15.5 0.2        11.9    0.752 good                                                       none    __________________________________________________________________________

                                      TABLE 9    __________________________________________________________________________           Draw ratio                        Relaxation in           First stage of                 Second stage of             longitudinal           longitudinal                 longitudinal                          Transverse                               Longitudinal                                       Transverse                                             direction           stretching                 stretching                          stretching                               re-stretching                                       re-stretching                                             (%)    __________________________________________________________________________    Example 28           2.2   2.6      3.5  1.5     1.3   1.0    Example 29           2.0   3.3      3.3  --      1.8   0.5    Example 30           2.0   2.2      3.8  1.6     --    0.5    __________________________________________________________________________                                Peak        Property                                intensity   for capacitor    F-5 value        Elongation ratio R in                                      Amorphous                                            Dielectric                                                 Break-    (kg/mm.sup.2)    at breakage                                laser Raman                                      orientation                                            loss down          Longitudinal                Transverse                     Longitudinal                           Transverse                                scattering                                      coefficient                                            at 105° C.                                                 voltage          direction                direction                     direction                           direction                                (I.sub.MD /I.sub.ND)                                      f.sub.MD                                            (%)  (V/μm)    __________________________________________________________________________    Example 28          20.0  19.8 43    75   15.6  0.823 0.98 610    Example 29          19.3  20.0 65    85   14.8  0.799 1.06 500    Example 30          20.5  17.3 40    92   16.2  0.843 1.12 550    __________________________________________________________________________

INDUSTRIAL APPLICATIONS OF THE INVENTION

In the biaxially oriented film according to the present invention,because the film has a specified orientation in the longitudinaldirection, the irregularity in properties of the film such asirregularity in thickness and irregularity in birefringence is small aswell as the strength of the film in the longitudinal direction is great.Therefore, the film is useful particularly as a base film for magneticrecording media, heat transfer ribbons, capacitors and thermo-stencilprinting plates which require thin films.

We claim:
 1. A biaxially oriented polyester film characterized in thatit has a Raman intensity ratio (R), of 6 or more, said Raman intensityratio being defined by the relationship R=I_(MD) /I_(ND) wherein I_(MD)is the peak intensity in the longitudinal direction of said film, and(I_(ND)) is the peak intensity in the thickness direction of the film,as determined at 1615 cm⁻¹ by the laser Raman scattering method.
 2. Abiaxially oriented polyester film according to claim 1, said film havinga molecular chain, wherein the amorphous orientation coefficient f_(MD)of said molecular chain in the longitudinal direction of said film is0.5 or more.
 3. A biaxially oriented polyester film according to claim 2wherein said film has a F-5 value in the longitudinal direction of saidfilm, which is 15 kg/mm² or more.
 4. A biaxially oriented polyester filmaccording to claim 1, wherein said film has an F-5 value in thelongitudinal direction of said film, which is 15 kg/mm² or more.
 5. Abiaxially oriented polyester film according to claim 1, wherein thepolyester of said film is at least one selected from the groupconsisting of polyethylene terephthalate, polypropylene terephthalate,polyethylene isophthalate, polyethylene naphthalate, and a copolymerthereof.
 6. A biaxially oriented polyester film according to claim 1,having a concentration of COOH end groups in said film wherein saidconcentration of COOH end groups in said film is in the range of 15-80eq/10⁶ g.
 7. A biaxially oriented polyester film according to claim 1,said film having particles, wherein said particles are contained in saidfilm at a content of 0.01-10% by weight.
 8. A biaxially orientedpolyester film according to claim 6, wherein said particles are at leastone selected from the group consisting of titanium oxide, silicon oxide,aluminum oxide, zirconium oxide, kaolin, talc, calcium phosphate,calcium carbonate, carbon black and organic particles.
 9. A biaxiallyoriented polyester film according to claim 1, wherein said film isformed as a laminated film, said laminated film having at least twolayers, wherein at least one layer of said laminated film has athickness (t), and contains particles, said particles having a meanparticle diameter (d), and wherein said layer of said laminated film hasa ratio (d/t) of 0.1-10.
 10. A biaxially oriented polyester filmaccording to claim 1, wherein said film has an F-5 value in thetransverse direction of said film of 12 kg/mm² or more, and said film isused for magnetic recording media.
 11. A biaxially oriented polyesterfilm according to claim 1 wherein said film has a total thickness in therange of 2-8 μm; a surface roughness (Ra) in the range of 0.1 to 5 nm;and said film is used for coated magnetic recording media used fordigital video tape recorders.
 12. A biaxially oriented polyester filmaccording to claim 1, wherein said film has a total thickness of 3-9 μm;a surface roughness (Ra) in the range of 0.1 to 5 nm; and said film isused for evaporated metal magnetic recording media used for digitalvideo tape recorders.
 13. A biaxially oriented polyester film accordingto claim 1, wherein said film has a heat shrinkage in the transversedirection of said film of 3% or less, wherein said heat shrinkage ismeasured at 100° C. for 30 minutes, and said film is used for heattransfer ribbons.
 14. A biaxially oriented polyester film according toclaim 1, wherein said film has a tensile elongation at break of saidfilm in the longitudinal direction of said film of 100% or less, andsaid film is used for capacitors.
 15. A biaxially oriented polyesterfilm according to claim 1, wherein said film has a tensile elongation atbreak of said film in the transverse direction of said film of 200% orless, and said film is used for capacitors.
 16. A biaxially orientedpolyester film according to claim 1, wherein said film has a heat offusion (Δ Hu) of 12 cal/g or less, and said film is used forthermo-stencil printing plates.
 17. A process for producing a biaxiallyoriented polyester film comprising the steps of:casting a cast filmhaving a plurality of edge portions and a central portion, said edgeportions having a thickness and said central portion having a thickness;controlling a ratio (A/B) of the maximum thickness of an edge portion ofa cast film (A) to the thickness of a central portion in the transversedirection of a cast film (B) in the range of 2 to 6; stretching saidcast film biaxially; and controlling the Raman intensity ratio (R) ofthe biaxially oriented film to be 6 or more, said Raman intensity ratiobeing defined by the relationship R=I_(MD) /I_(ND) wherein (I_(MD)) isthe peak intensity in the longitudinal direction of the film, and (IND)is the peak intensity in the thickness direction of the film determinedat 1615 cm⁻¹ by the laser Raman scattering method.
 18. A process forproducing a biaxially oriented polyester film according to claim 17,wherein said stretching of said cast film is accomplished by performinga first axial stretching and a second axial stretching, wherein saidfirst axial stretching is performed in the longitudinal direction ofsaid film in two or more stages, and thereafter a second axialstretching is performed in the transverse direction.
 19. A process forproducing a biaxially oriented polyester film according to claim 18,wherein said first axial stretching is comprised of a first-stagelongitudinal stretching and a second-stage longitudinal stretching, saidfirst-stage longitudinal stretching is performed at a temperature of100° to 120° C. at a draw ratio of 1.5 to 2.5 times, said second-stagelongitudinal stretching is performed at a temperature of 70° to 98° C.,and thereafter, said second axial stretching is performed at a drawratio of 3 to 6 times.